Method for manufacturing cold rolled steel having excellent drawability

ABSTRACT

A method for producing cold rolled drawing quality steels containing less than 0.01% C and less than 0.50% Mn in which an inert or decarburizing gas is blown through a molten steel which is then vacuum degassed, cast either into ingots or continuously cast, hot and cold rolled and then subjected to a rapid continuous anneal at a temperature of at least 650*C. When the aluminum-killed steel is produced, aluminum is added to said molten steel in a sufficient amount to provide more than 0.02% soluble aluminum in the steel, and the hot rolled steel is coiled at a temperature above 630*C to precipitate aluminum nitride.

United States Patent 1191 Kubotera et al. I

1 June 28, 1-974,

[ 1 METHOD FOR MANUFACTURING COLD ROLLED STEEL HAVING EXCELLENT DRAWABILITY 22 Filed: Dec.24, 1970 211 Appl. No.: 101,317

30 Foreign Application Priority Data Dec. 27. 1969 Japan 44-104969 [52] US. Cl l48/2,75/49, 148/12 [51] Int. Cl. C2111 7/02, C21d 9/48 [58] Field of Search 148/2, 3,12, 12.], 31,

[56] References Cited 4 UNITED STATES PATENTS 2,993,780 7/1961 Allard 75/49 3,178,318 4/1965 Shimizu et a1. 148/2 3,183,078 5/1965 Ohtake et a1. 75/129 X 3,208,844 9/1965 Kato 1. 75/49 3,248,270 4/1966 Laidman et a1 148/12 3,392,009 7/1968 Holmes 75/59 3,417,463 12/1968 Knuppel et a1 75/49 UX 3,459,537 8/1969 Hornak 75/49 3,522,110 7/1970 Shimizu et a1. 148/12 OTHER PUBLICATIONS Journal of Metals, Vol. 22, No. 8, August 1969 pages 68-73.

Trans. of ASM, Vol. 58, 1965 pages 672- -686. V Hultgren, Paper Presented at Chicago Reg. Tech. Meeting, Amer. Iron & Steel Inst, Nov. 9, 1967, pages 1-20.

Richards, Special Report 79, The Iron and Steel lnstitute, London, 1963 pages 22-33. V

Dewsnap, An Investigation of the Rapid Annealing of Sheet Gauge Mild Steel, Special Pat. No. 79, 1151 pages 112 -120.

Primary Examiner-Charles N. Lovell Attorney, Agent, or Firm-Flynn and Frishauf 571 ABSTRACT A method for producing cold rolled drawing quality steels containing less than 0.01% C and less than 0.50% Mn in which an inert or decarburizing gas is blown through a molten steel which is then vacuum degassed, cast either into ingots o'r continuouslycast, hot and cold rolled and then subjected to a rapid continuous anneal at a temperature: of at least 650C. When the aluminum-killed steel is produced, aluminum is added to said molten steel in a sufficient amount to provide more than 0.02% soluble aluminum in the steel, and the hot rolled steel is coiled at a temperature above 630C to precipitate aluminum nitride.

4 Claims, 2 Drawing Figures METHOD FOR MANUFACTURING COLD ROLLED STEEL HAVING EXCELLENT DRAWABILITY The present invention relates to a method for manufacturing cold rolled drawing quality steel which combines a specific degassing method, hot rolling conditions and a short rapid annealing step such as continuous annealing.

In the prior art, cold rolled steel sheets for drawing are generally manufactured by heating sheets for a long period of time in a box annealing furnace.

However, the conventional method requires a long annealing time and considerable space for the annealing furnace installation. Further, it was impossible to heat the entire length of a coil uniformly and obtain a uniform quality therethrough. On the other hand, the continuous annealing method has such advantages as high annealing efficiency per unit, and uniformity in quality in the direction of the coil length. At the same time the continuous anneal method has drawbacks such as insufficient grain growth due to rapid heating and short soaking timeand it is impossible to achieve grain growth in the direction advantageous for deep drawing. Further, precipitation of carbon as carbides in the steel is not possible due to rapid quenching after soaking. The inferior qualities of the steel such as an increase in yield stress and aging properties and a decrease in total elongation is noticeable. Accordingly, the continuous annealingmethod has conventionally only been applied for manufacturing hard hot cold rolled steel sheets for plates for tin plating rather than for steel sheets for deep drawing. Thus, it would be of great advantage to the art if manufacture of cold rolled steel sheets for drawing having properties such as improved annealing efficiency and uniformity in the quality of the steels normal with the continuous annealing method, could be achieved.

The present invention was developed to provide such a method asaforementioned after various researches.

it is an object of the present invention to reduce as much as possible elements which are in solution at the completion of hot rolling, and moreover, to facilitate grain-growth in hot rolled steel sheets by means of a degassing device. It is desirable to decrease unnecessary elements as much as possible before annealing in order to (a) decrease soluble atoms which if present would precipitate after continuous annealing and quenching and (b) make the grain size of the hot rolled sheet as large as possible since the grain size after continuous annealing greatly depends upon that before cold rolling.

it is hereinafter described more in detail reference being made to the accompanying drawing in which FIG. 1(a) and FIG. 1(b) represent cross sectional views of the lower part of two degassing apparatus used in the present invention.

The present invention aims to provide a method for manufacturing cold rolled steel sheets for drawing wherein a specific degassing method is performed wherein inert gas or the like is blown through porous bricks arranged at the bottom of the ladle shown in FIGS. la and lb to facilitate reactions a conventional degassing is performed followed by conventional hot rolling, cold rolling, and a continuous anneal.

For a better understanding of the present invention,

Referring to FIGS. 1a and lb, porous bricks 2 are arranged at the bottom of ladle 1 and inert gas or other I gas facilitating a decarburizing reaction is supplied from a gas cylinder 3 through the porous bricks 2 and othergas supplying ports.

Using conventional degassing methods production of steel having a carbon content of less than 0.010 percent is difficult and the degassing velocity is slow. Considerable time required to obtain steels containing less than 0.010 percent of C and it is difficult to stably decrease C content to less than 0.010 percent. However, in accordance with the method of the present invention mentioned above, the generation of gas bubbles 4 is facilitated and steels containing the foregoing amount of C can be obtained easily within a short period of time.

The following table 1 shows the results of an experiment wherein the low carbon steel was melted by means of a t, converter the amount of oxygen in the molten steel being slightly adjusted with aluminum 20 gassing method in which argon gas was blown through after tapping and the steel then being treated by a dethe porous bricks arranged at the bottom of the ladle mentioned above.

.Note:Samples 1 through 3 were treated by the degassing method of the present invention. 4 through 6 by a conventional degassing method.

As is clear from the above table 1, it is possible to decrease C content remarkably within a short period of time by way of combining the conventional degassing methodand such a method of blowing gas as aforementioned.

The molten steel degassed is either made into an ingot or continuously cast after adding alloying additions such as Mn, Al or the like according to requirements after the desired degree of degassing is attained.

Example 1 An example of continuous annealing tests upon steels selected from the steels tapped and treated by the present and conventional degassing methods is shown.

The following table 2 shows the result of the check analysis of hot rolled steel sheets. Seven kinds of the steels, A, B, C, F, G, H and I in table 2 were obtained by the degassing treatment.

The steels A and F among said steels were processed into degassed ultra-low carbon steels by way of the specific degassing method mentioned above. The steels A, B and C were tapped after degassing without using such a deoxidizer as Al, Ti or the like. Further, the steels F, G, H and I were deoxidized with Al after the desired degree of degassing was attained. Conventional rimmed steel (steel D) andaluminum killed steel (steel 1) are for comparative purposes.

below 570C, respectively.

vSol Al I as [Ill

' The following table fi shows tl iehot rolli ri gcondition s applied for these steels. Since AlN cannot be precipitated from Aluminum killed steel during the continuous annealing operation, two coiling temperature levels were applied to said steels to determine the effects thereof.

Table 3 Hot rolling conditions Steel Finishing temperature Coiling temperature A 895C 630C B 870 650 C 880 670 D 875 640 F 1 910C 700C All the hot rolled steel sheets have the thickness 2.6mm and were cold rolled to 0.5mm thick. (Cold reduction rate: 80.8 percent) The continuous annealing conditions are as follows:

Line speed 80 m.p.m.

Temperature in the Maintenance time heating zone 900930C in the furnace 60 secs. Temperature in the soaking zone 740-780C 60 secs. Temperature at the SOD-520C 80 secs. inlet to a slow cooling zone I Temperature at the 400-430C 80 secs.

outlet from a slow cooling zone The temper rolling rate was 0.8 percent to 1.5 percent. The following table 4 shows the mechanical test values of the steels in table 3.

Table 4-l below shows the results of the standard metallurgical test upon the steels without addition of such a deoxidizer as Ti, Al or the like. Table 4-2 shows the results of tests upon aluminum killed steels to which Al was added after the desired degree of degassing is 55 attained, then hot rolled and coiled at high temperature ranging from 670C to 700C and Table 4-3 shows the test results on said steels coiled at temperatures of According tothese results, the steels to which such a deoxidizer as Al, Ti or the like was not added were inferior in quality in proportion to the increase of C content. The properties for an ordinary 'cold rolled steel sheet for drawing can be obtained only in steels containing less than 0.010 percent of C. Such a steel is characterized in that although the properties such as the yield point, the aging properties are relatively inferior, it has remarkable drawability (Lankford value) as compared with aluminumkilled steels of Tables 4-2 and 4-3.

I The results reported in Table 4-3 establish that AlN is not sufficiently precipitated from aluminum killed steel hot rolled and coiled at the temperature of less than 570C, so that the aging properties of said steel is inferior and further the qualities thereof as an ordinary cold rolled steel sheet for drawing are poor. Steels hot rolled and coiled at temperatures of over 650C show great improvement in qualities.

In this case the influence of C content is also apparently seen. The steels H and l in which C content is not decreased sufficiently show great inferiority in aging properties and total elongation, so that it may be said that it is necessary to reduce the carbon content to less than 0.010 percent by the degassing process. That is to say, cold rolled steel sheets having excellent drawability can be manufactured by continuously annealing aluminum killed steels after decreasing C content thereof to less than 0.010 percent and finishing hot rolling and coiling at temperatures sufficient to precipitate aluminum nitride.

It was proved from other tests that the precipitation of AlN greatly depends upon the S01 Al content in steel and that a steel containing more than 0.020 percent of 801 Al requires the coiling temperature to be over 630C.

It is preferable that the elements existing as soluble elements (C, N, Mn, A1 or the like) and impurities in the steel are as low as possible in view of grain-growth both at the time of high temperature coiling and the process of continuous annealing. Moreover, it is desirable that the finishing temperature of hot rolling is made as high as possible.

The steels of the present invention can be continuously cast. Said steels tapped show no generation of gas bubbles during casting and show a similar solidrfica tion of that of killed steels.

Table 4-1 (thickness of steel sheet: 0.5 mm) Aging Yield Yield Tensile Total index point elon ation strength elongation Erichsen numbers Mark (kg/mm) (50) (kg/mm) (mm) (kg/mm) A 20.8 0 30.8 47.8 10.8 6.4 B 23.7 0 32.6 43.4 10.4 6.6 C 25.4 0 34.8 39.8 9.8 7.3 D 26.1 0 38.| 35.6 8.7 7.2

Table 4-2 4 (thickness of steel sheet: 0.5 mm) Aging Yield Yield Tensile Total index point elongation strength elongation Erichsen r numbers Mark (kg/mm (kg/mm (mm) (kg/mm) "T5613 l W (thickness of steel sheet: 0.5 mm) Aging Yield 7 Yield Tensile Total index point elongation strength elongation Erichsen r numbers Mark (kg/mm) (kg/mm) (mm) (kg/mm) Thus, according to the 6563261 invention, is possimolten steel to provide said steel with a soluble alumible to manufacture cold rolled steel sheets for drawing num content of more than 0.02 percent; casting said which have excellent and uniform properties, with high molten steel; hot rolling said steel, and then coiling at annealing efficiency. Moreover, the present invention a temperature above 630C whereby the aluminum nican be applied in wide fields, for example, for manufactride-formed by reaction of said soluble aluminum with turing steel sheets for surface treatment for drawing. nitrogen in said steel is precipitated; cold rolling said The embodiments of the invention in which an exclusteel; and then rapidly annealing said steel at a tempersive property or privilege is claimed are defined as folature of about 650C in a continuous annealing apparalows: tug, g l. A method for manufacturing aluminum-killed cold 2. The method of claim 1 wherein said molten steel rolled sheet steel containing less than 0.01 percent C is cast as an ingot. and less than 0.40 percent Mn having excellent drawing 3. The method of claim 1 wherein said molten steel properties comprising vacuum degassing molten steel is continuously cast. containing less than 0.40 percent Mn and blowing (i) 4. The method of claim 1 wherein said molten steel an inert gas or (ii) a decarburizing gas through said is blown with an inert gas. molten steel to provide a steel containing less than 40 i V A r 0.010 percent C; adding sufficient aluminum to said 

2. The method of claim 1 wherein said molten steel is cast as an ingot.
 3. The method of claim 1 wherein said molten steel is continuously cast.
 4. The method of claim 1 wherein said molten steel is blown with an inert gas. 